This invention relates to a stabilized aromatic polyester composition and, more specifically, to a process for incorporating a phosphite stabilizer into an aromatic polyester composition.
The aromatic polyester used in this invention is obtained from terephthalic acid and/or functional derivatives thereof; isophthalic acid and/or functional derivatives thereof; or mixtures thereof (with the terephthalic acid/isophthalic acid unit mole ratio being about 9:1 to about 1:9) and a bisphenol of the following general formula (I) ##STR1## wherein --X is selected from the group consisting of --O--,--S--, --SO.sub.2 --, --SO--, --CO--, an alkylene group containing 1 to 4 carbon atoms, and an alkylidene group containing 1 to 4 carbon atoms, and R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.1 ', R.sub.2 ', R.sub.3 ', and R.sub.4 ', which may be the same or different, each represents a member selected from the group consisting of a hydrogen atom, a chlorine atom, a bromine atom and an alkyl group containing 1 to 4 carbon atoms, or functional derivatives thereof.
Known methods for producing such an aromatic polyester include, for example, an interfacial polymerization method which involves mixing an aromatic dicarboxylic acid chloride dissolved in a water-immiscible organic solvent with an alkaline aqueous solution of a bisphenol, a solution polymerization method which comprises heating a bisphenol and an acid chloride in an organic solvent, and a melt polymerization method which comprises heating a phenyl ester of an aromatic dicarboxylic acid and a bisphenol, e.g., as disclosed in U.S. Pat. Nos. 3,884,990 and 3,946,091.
It is also known that aromatic polyesters derived from aromatic dicarboxylic acids and bisphenols have superior properties to polyesters derived from aromatic dicarboxylic acids and aliphatic alkylene glycols. Specifically, these polyesters have many superior characteristics, e g., mechanical properties such as tensile strength, elongation, flexural strength, flexural recovery and impact strength, heat-distortion temperature, dimensional stability, electrical properties, and fire retardancy. Because of these superior properties, these polyesters are known to be useful in a wide range of fields as various molded articles, films, fibers, and coating materials obtained by extrusion molding, injection molding, etc.
The aromatic polyesters obtained by the above methods frequently assume yellow or brown colors in their as-prepared state. It is also noted that aromatic polyesters turn yellow when heated during injection molding, extrusion molding and other shaping operations which are used to form ordinary molded products or films. Such coloration is extremely inconvenient when a lack of color in the resulting molded articles is important. Furthermore, when a pigment is incorporated into a molding composition to obtain a desired color, the color of the final molded product frequently is quite different from the desired color.
The aromatic polyesters used in this invention have a high heat distortion temperature, and therefore, are frequently used under high temperature conditions. The above-described coloration, however, proceeds generally under high temperature conditions and impairs the transparency of the molded product. Consequently, such a product cannot be used in applications which require transparency and a lack of color at high temperatures.
Such a coloration is also considered to occur with the decomposition of the polymer. Thus, coloration is synonymous with an irregular reduction in the logarithmic viscosity number (intrinsic viscosity) of the polymers constituting the molded articles. Coloration and reduction of intrinsic viscosity deteriorates the useful properties of these polymers and are therefore extremely disadvantageous in obtaining products which are uniform.
It has been suggested to incorporate organophosphorus (phosphite and phosphate) compounds into aromatic copolyester compositions in order to reduce coloration and depolymerization as in U.S. Pat. No. 4,123,420. However, as far as incorporating P-containing stabilizers into melt polymerized aromatic polyesters, there has been no indication relative to the importance of when such stabilizer should be incorporated into the polymer. While it appears best to add the stabilizer at the beginning of polymerization in order to have the antioxidant advantage as soon as possible, it has been found that if there is any moisture present, the phosphate or phosphite esters hydrolyze and form free acid which interferes with the polymerization catalyst and subsequent molecular weight buildup of the polymer.